The invention discloses a single tube fin type heat exchanger and assembling thereof. The heat exchanger assembly comprises a liquid flow pipe and a plurality of pairs of smoke fins, wherein at least the middle of the liquid flow has the ellipse section, the length of the long axis of the ellipse is 2a, and the length of the short axis of the ellipse is 2b; the multiple pairs of smoke fins are matched with the outer surface of the single liquid flow pipe in shape and connected to the outer surface of the single liquid flow pipe; the multiple pairs of smoke fins are arranged at intervals in the axial direction of the single liquid flow pipe; each pair of smoke fins comprises the first smoke fin and the second smoke fin which are arranged in an aligned mode and jointly form a rectangular outline, the shape of the first smoke fin and the shape of the second smoke fin are basically the same, and outer side gaps are formed between the first smoke fin and the second smoke fin of each pair of smoke fins in the long axis direction of the ellipse; the length of the long edges of each rectangle is A, the length of the short edges of each rectangle is B, and the ratio of A to 2a is within 1.3 to 2.3; and the ratio of B to 2b is within 2.0-4.0.